This application claims priority to prior application JP 2002-158526, the disclosure of which is incorporated herein by reference.
The present invention relates to a noise filter that is mounted in an electronic device or electronic equipment for removing noise generated therein.
Digital technologies are important technologies supporting IT (Information Technology) industries. Recently, digital circuit technologies such as LSI (Large Scale Integration) have been used in not only computers and communication-related devices, but also household electric appliances and vehicle equipment.
High-frequency noise currents generated in LSI chips or the like do not stay in the neighborhood of the LSI chips but spread over wide ranges within mounting circuit boards such as printed circuit boards, and are subjected to inductive coupling in signal wiring or ground wiring, thereby leading from signal cables or the like as electromagnetic waves.
In those circuits each including an analog circuit and a digital circuit, such as a circuit in which part of a conventional analog circuit is replaced with a digital circuit, or a digital circuit having analog input and output, electromagnetic interference from the digital circuit to the analog circuit has been becoming a serious problem.
As a countermeasure therefor, a technique of power supply decoupling is effective wherein an LSI chip as a source of generation of high-frequency current is separated from a dc power supply system in terms of high frequencies. Noise filters such as bypass capacitors have been used hitherto as decoupling elements, and the operation principle of the power supply decoupling is simple and clear.
The capacitors as noise filters used in conventional ac circuits form two-terminal lumped constant noise filters and solid electrolytic capacitors, electric double-layer capacitors, ceramic capacitors or the like are often used therefor.
When carrying out removal of electrical noise in an ac circuit over a wide frequency band, inasmuch as a frequency band that can be dealt with by one capacitor is relatively narrow, different kinds of capacitors, for example, an aluminum electrolytic capacitor, a tantalum capacitor and a ceramic capacitor having different self-resonance frequencies, are provided in the ac circuit.
Conventionally, however, it has been bothersome to select and design a plurality of noise filters that are used for removing electrical noise of a wide frequency band. In addition, there has been a problem that, because of using different kinds of the noise filters, the cost is high, the size is large, and the weight is heavy.
Further, as described above, for dealing with higher-speed and higher-frequency digital circuits, there have been demanded those noise filters that can ensure decoupling over a high frequency band and exhibit low impedances even in the high frequency band.
However, the two-terminal lumped constant noise filters have difficulty in maintaining low impedances up to the high frequency band due to self-resonance phenomena of capacitors, and thus are inferior in performance of removing high-frequency band noise.
Further, the electronic equipment or devices with the LSI chips or the like mounted therein have been required to be further reduced in size, weight and cost. Therefore, the noise filters that are used in those electronic equipment or devices have also been required to be further reduced in size, to be structured more simply, and to be manufactured more easily.
Therefore, it is an object of the present invention to provide a transmission line type noise filter that is excellent in noise removing characteristic over a wide band including a high frequency band and that has a small size and a simple structure.
A transmission line type-noise filter according to the present invention has a central conductor, an external conductor, and a dielectric formed between the central conductor and the external conductor, and is characterized by comprising a metal fine wire made of valve-operational metal and serving as the central conductor; a capacitance forming portion formed over a predetermined length on the periphery of the metal fine wire; and a conductor layer formed on a surface of the capacitance forming portion and serving as the external conductor, wherein the capacitance forming portion comprises a sintered body formed on the metal fine wire and made of the foregoing valve-operational metal; a dielectric film formed on a surface of the sintered body and serving as the dielectric; and a solid electrolyte layer formed on a surface of the dielectric film.
The sintered body may be formed by press-molding powder of the valve-operational metal, then sintering it at a predetermined temperature, or may be formed by winding a green sheet formed from slurry including powder of the valve-operational metal, around the metal fine wire as a core, then sintering it at a predetermined temperature.
The dielectric film may be an oxidized film of the valve-operational metal.
The valve-operational metal may be tantalum or niobium.
The solid electrolyte layer may be formed by a single conductive high molecular compound layer or a plurality of conductive high molecular compound layers.
The transmission line type noise filter may further comprise a first and a second anode terminal connected to both ends of the metal fine wire, respectively; a cathode terminal connected to the conductor layer; and a resin package covering the metal fine wire, the capacitance forming portion and the conductor layer, excluding at least portions of the first and second anode terminals and the cathode terminal, respectively, thereby to be formed as an electronic element.
Other objects, features and advantages of the present invention will become apparent from the following description of this specification.